Need cure for Sunfire True Subwoofer hum

My True subwoofer performs admirably but from time to time it emits a very pronounced hum. Any Cures?
Teach it the words?
I used to give mine a good whack every now and then, which would fix it for awhile, but not very long. Eventually I succumbed and sent it back to Sunfire. They fixed it and upgraded it to the equivalent of a Mark IV for about $200+ if I recall correctly.
I sent mine back to sunfire for the upgrade. It was free + shipping back then. The hum is inherent in the original.
I used a cheater plug on mine (so I could invert polarity) it helped somewhat.